Transfer function photoelectrochemical

The terminology used for the transfer function requires some care. If the input function is an intensive quantity such as voltage (also called an across function), and the output function is an extensive quantity such as current (also called a through function), the ratio of output to input can be referred to as an admittance. If the type of input and output functions are reversed, then the ratio becomes an impedance. If the input and output functions are both of the same type, the ratio of output to input can be referred to as a gain function. However, these conventions, which are usually employed in network analysis, have not always been followed consistently in the development of new photoelectrochemical techniques. For example, the frequency dependent photocurrent efficiency, O, shown in Table 1 is often referred to as the opto-electrical admittance and its inverse as the opto-electrical impedance [62, 64-66], in spite of the fact that it is the ratio of two through functions. It would be preferable to use the term opto-electrical transfer function. The inverse of has also been called the photoelectrochemical impedance [53, 70]. To avoid confusion, the use... 

IMPS uses modulation of the light intensity to produce an ac photocurrent that is analysed to obtain kinetic information. An alternative approach is to modulate the electrode potential while keeping the illumination intensity constant. This method has been referred to as photoelectrochemical impedance spectroscopy (PEIS), and it has been widely used to study photoelectrochemical reactions at semiconductors [30-35]. In most cases, the impedance response has been fitted using equivalent circuits since this is the usual approach used in electrochemical impedance spectroscopy. The relationship between PEIS and IMPS has been discussed by a number of authors [35, 60, 64]. Vanmaekelbergh et al. [64] have calculated both the IMPS transfer function and the photoelectrochemical impedance from first principles and shown that these methods give the same information about the mechanism and kinetics of recombination. Recombination at CdS and ZnO electrodes has been studied by both methods [62, 77]. Ponomarev and Peter [35] have shown how the equivalent circuit components used to fit impedance data are related to the physical properties of the electrode (e.g. the space charge capacitance) and to the rate constants for photoelectrochemical processes. 

It can be concluded that measurement of the optoelectrical transfer function in a photoelectrochemical cell is a powerful technique for studying the mechanism and kinetics of photoinduced electron transfer. The technique has been further exploited... 

Song, H. and Macdonald, D.D. (1991) Photoelectrochemical impedance spectroscopy I. Validation of the transfer function by Kramers-Kronig transformation. Journal of The Electrochemical... 

In principle, like all electrochemical reactions initiated by the transfer of an electron across an electrode-electrolyte interface, photoelectrochemical transformations offer the possibility of more precise control than can be attained with reactions that take place in homogeneous solution [62, 63]. This better selectivity derives from three features associated with reactions that take place on surfaces, and hence with the photoelectrochemical event the applied potential (allowing for specific activation of a functional group whose oxidation potential is higher, even in a multifunctional molecule) the chemical nature of the electrode surface (and hence of the adsorption equilibrium constant of a specific molecule present in the double layer) and, finally, control of current flow (and hence a constraint on the number of electrons passed to an adsorbed reactant). 

Because the sensitizing dye itself does not provide a conducting functionality but is distributed at an interface in the form of immobilized molecular species, it is evident that for charge transfer, each molecule must be in intimate contact with both conducting phases. It is clear that this applies to the porous wide band gap semiconductor substrate into which the photoexcited chemisorbed molecules inject electrons. It is also evident that in the photoelectrochemical form of the sensitized cell, the liquid electrolyte penetrates into the porosity, thereby permitting the intimate contact with the charged dye molecule that is necessary for charge neutralization after... 

In the presence of redox couples confined to the hydrophobic liquid phase, photoinduced heterogeneous electron transfer can be effectively monitored by photoelectrochemical techniques under potentiostatic conditions. The photocurrent responses are uniquely related to specifically adsorbed porphyrins, as demonstrated by the photocurrent anisotropy to the angle of polarisation of the incident illumination (Section 4.3). Systematic studies of the photocurrent intensity as a function of the formal potential of the redox couple and the Galvani potential difference revealed that the dynamics of electron transfer are determined by the distance separating the redox species at the interface. Other processes including decay of the electronically excited state, back electron transfer, porphyrin regeneration and coupled ion transfer play important role on the dynamics of the photocurrent responses. 

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